This invention relates to fusible elements for electric fuses, in particular time-lag or time-delay fuses.
There are many methods to delay blowing of electric fuses at relatively high inrush currents of short duration. One of these methods consists in effecting mutual heating of different surfaces of a fusible element. For instance, the perforated center portion of a fusible element having a relatively low fusing i.sup.2 .multidot.t value, e.g. silver or copper, may be folded longitudinally to form a plurality of longitudinal edges. The surfaces to different sides of these edges have a mutual heating effect upon each other.
Another method to achieve time-lag in an electric fuse is to place a metal-serving element having a fusing point less than the fusing point of the base metal on which it is affixed, e.g. silver or copper, at the location where the highest temperature of the metal prevails. This effects a derating of the fuse and a concomitant increase of its time lag.
Still another method to achieve time-lag is to increase the temperature of the center portion of the fusible element of the fuse either by thermally insulating the latter from the terminal elements by so-called heat dams, or by converting such heat dams into generators of heat by imparting a sufficiently high resistance to them.
The invention is predicated on a combination of the aforementioned methods.
A particular object of this invention is to meet a certain U.L. Standard by a fusible element that is predicated on the application of a metal severing element, or overlay, having a fusing point lower than the fusing point of the base metal by which the metal severing element is supported, or which supports the overlay. Fuses whose fusible elements are severed in a certain current range by a metal diffusion process are known as M-effect fuses, and this invention refers to M-effect fuses.
The above referred-to U.L. Standard requires that a time-lag fuse must fuse, or blow, within 1 hour at currents equal to 135% of their respective current rating. This requirement causes difficulties, particularly if the fusible element is relatively short, such as in fuses rated 30 amps. at 250 volts, because under the conditions specified by this U.L. Standard the temperature to cause the M-effect to occur can hardly be reached. Substitution of a high specific resistance metal such as bronze for a low specific resistance metal such as silver or copper, results in unacceptably high current-carrying temperatures of the fuse and in metal masses of the fusible element which fuse too slowly under short-circuit conditions, and which evolve too large amounts of metal vapors ro be acceptable.
There are many M-effect fusible elements known in the art which comply with the above U.L. Standard, but these fusible elements are complex and of one single metal and this single metal must be silver if compliance of the above U.L. Standard is to be coupled with small peak let-through currents. It is, therefore, a further object of this invention to provide fusible elements for time-lag fuses that comply with the above U.L. Standard, do not run too hot, are capable of effectively interrupting high short-circuit currents and whose current path comprises a plurality of different serially related metals.
Another object of the invention is to provide time-lag fuses that are current-limiting under short-circuit conditions and that include particular means for derating the fuses by mutual heating of sections of the fusible element thereof, as will become more apparent as this specification proceeds.